Method and apparatus for argon recovery in a cryogenic air separation unit integrated with a pressure swing adsorption system

ABSTRACT

A method and apparatus for argon recovery in which an impure argon stream is separated from air within a cryogenic air separation unit having a divided wall argon rejection/rectification column. The resulting argon stream is subsequently recovered and purified within an integrated pressure swing adsorption system to produce product grade argon.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is claims the benefit of and priority to U.S.provisional patent application Ser. No. 62/199,460 filed on Jul. 31,2015, the disclosure of which is incorporated by reference herein.

TECHNICAL FIELD

The present invention is related to a method and apparatus for argonrecovery in which argon is separated from air within a cryogenic airseparation plant having a divided wall argon rejection column andcondensed using an argon condenser disposed internally within the lowerpressure column to form a liquid and/or gaseous argon stream. The liquidor gaseous argon stream is subsequently recovered and purified within anintegrated adsorbent based argon refining and purification subsystem toproduce product grade argon.

BACKGROUND

Argon is a highly inert element used in the some high-temperatureindustrial processes, such as steel-making. Argon is also used invarious types of metal fabrication processes such as arc welding as wellas in the electronics industry, for example in silicon crystals growingprocesses. Still other uses of argon include medical, scientific,preservation and lighting applications.

While argon constitutes only a minor portion of ambient air (i.e. 0.93%by volume), it possesses a relatively high value compared to the oxygenand nitrogen products that are also recovered from air separationplants. Argon is typically recovered in a Linde-type double columncryogenic air separation arrangement by extracting an argon rich vapordraw from the lower pressure column and directing the stream to a“superstaged” column or crude argon column to recover the argon. Thisargon distillation process typically includes an argon condensing unitsituated above the argon column. The argon condensation load istypically imparted to at least a portion of the oxygen rich columnbottoms or kettle stream prior to its introduction into the lowerpressure column. Argon can be produced directly by this “superstaged”distillation process to merchant liquid purities (e.g. about 1000 ppm to1 ppm oxygen) in roughly 90 to 180 stages of separation or produced tointermediary purities (e.g. about 15% to 1% oxygen) in roughly 20 to 50stages of separation. In some applications, the intermediate purityargon is then often subsequently refined by catalytic oxidation processemploying hydrogen.

Modern air separation plants almost exclusively employ a superstageddistillation process for high purity argon recovery. Drawbacks of thetypical three column argon producing air separation unit are theadditional capital costs associated with argon recovery and theresulting column and coldbox heights, often in excess of 200 feet, arerequired to recover the high purity argon product. As a consequence,considerable capital expense is incurred to attain the high purityargon, including capital expense for the separate argon columns,multiple coldbox sections, liquid reflux/return pumps, etc.

An alternative method of producing high purity argon is to take a lowerpurity argon-containing stream from an air separation plant and purifythe argon-containing stream using an adsorbent based purificationsystem. There have been combinations of cryogenic air separation unitsand adsorbent based purification systems with the objective to removeoxygen, nitrogen and other contaminants from the argon-containingstreams. See U.S. Pat. Nos. 4,717,406; 5,685,172; 7,501,009; and5,601,634; each of which are briefly described in the paragraphs thatfollow.

U.S. Pat. No. 4,717,406 discloses a liquid phase adsorption processwherein a feed stream from a cryogenic plant is directed to anadsorption based purification system. The adsorption based purificationsystem serves to purify the liquefied gas prior to introducing it into aliquid storage tank. The targeted applications include the removal ofwater and carbon dioxide from electronics grade gases and the disclosedregeneration method of the adsorbent beds is a temperature swingprocess.

U.S. Pat. No. 5,685,172 details a process targeting the removal of traceoxygen and carbon monoxide from a variety of inert gases. The processalso notes direct liquid processing and argon is cited as an examplefluid. Metal oxides (CuO, MnO2) are detailed as adsorbents for oxygen.Regeneration is accomplished through the use of a reducing gas such ashydrogen at modest temperatures (e.g., 150° C. to 250° C.). The use of areducing gas makes it difficult to integrate the adsorbent beds with theair separation units because the reducing gas is not made in the airseparation unit and but must be externally supplied to regenerate theadsorbents. More importantly, during regeneration of the adsorbent beds,argon rich fluids will be lost from the process.

U.S. Pat. No. 7,501,009 discloses a cyclic adsorption process for thepurification of argon. The process may be operated at cryogenictemperature while processing crude argon in the gaseous state. Zeolitesare noted as possible adsorbents for the disclosed pressure swingadsorption (PSA) system. Regeneration gas is directed back to theargon-oxygen rectification column.

U.S. Pat. No. 5,601,634 combines a typical cryogenic air separation unitand pressure swing adsorption (PSA) system in which both nitrogen andoxygen contained in the argon feed from the distillation column of thecryogenic air separation unit are removed in adsorbent beds.

All of the above-identified prior art solutions focus only onimprovements in the adsorbent based purification system of the combinedcryogenic air separation unit and adsorption based purificationarrangement and do not address improvements needed to the cryogenic airseparation unit, including the use of a divided wall argon rejectioncolumn and argon condenser disposed internally within the lower pressurecolumn, as contemplated in the present solution.

The use of divided wall columns within the prior art literature isclear, including some prior art references that teach the use of dividedwall columns for argon rejection. See, for example, U.S. Pat. Nos.8,480,860; 7,234,691; 6,250,106; 6,240,744; and 6,023,945. In addition,U.S. Pat. No. 5,114,445 teaches an improvement to the recovery of argonthrough the placement of an argon condenser within the lower pressurecolumn as part of a means to thermally link the top of the crude argoncolumn with the lower pressure column and which teaches that the mostsuitable location for the argon condenser is as an intermediate locationwithin the lower pressure column, particularly, the section of the lowerpressure column bounded by the feed point of the crude liquid oxygenbottoms from the higher pressure column and the vapor feed draw line forthe crude argon column.

Each of the above-identified prior art methods and systems, makeincremental improvements to the operating efficiency of cryogenic airseparation plants, and in some cases to the recovery of argon. However,each of the prior art references have notable short-comings or designchallenges that drive increased capital costs, plant configuration,and/or argon recovery inefficiencies. As a result, there is a continuingneed to develop further improvements to existing argon rejection andrecovery processes or arrangements that are fully integrated with thedistillation column and cycles of cryogenic air separation units. Inparticular, for some cryogenic air separation units there is a need todesign an argon rejection and recovery process within the air separationcycles that is flexible in that it avoids or defers some of the up-frontcapital costs associated with argon recovery but allows argon recoveryto be easily added to the cryogenic air separation unit at a later datewhen the argon production requirements change.

SUMMARY OF THE INVENTION

The present invention may be characterized as a method of producing apurified argon product in a cryogenic air separation unit integratedwith a pressure swing adsorption system, the method comprising the stepsof: (i) separating argon from an oxygen-argon containing stream within alower pressure column of the cryogenic air separation unit using anargon rectification column arrangement disposed within the lowerpressure column, the separation of the argon from the oxygen-argoncontaining stream producing an impure argon stream having between aboutand 4% and 25% of oxygen impurities; (ii) warming the impure argonstream to a temperature between about 200K and 300K; (iii) pressurizingthe impure argon stream to a pressure between 80 psig and 120 psig; (iv)purifying the impure argon stream by introducing the impure argon streaminto the pressure swing adsorption system, the pressure swing adsorptionsystem comprising an adsorbent bed having an adsorbent configured foradsorbing the oxygen impurities to produce a purified argon stream and awaste gas stream, wherein the adsorbent bed is subjected to analternating cycle having an on-line phase where the impure argon streamis purified within the adsorbent bed to produce the purified argonstream, and an off-line phase where the adsorbent contained in theadsorbent bed is regenerated and produces a waste gas stream; and (v)recycling the waste gas stream from the pressure swing adsorption systemback to the argon rectification column arrangement disposed within thelower pressure column wherein the impure argon stream consistsessentially of a portion of the ascending argon-rich vapor exiting theargon rectification column.

The present invention may also be characterized as an apparatus forproducing a purified argon product comprising: (a) a cryogenic airseparation unit having a higher pressure column and a lower pressurecolumn, and an argon rectification column arrangement disposed withinthe lower pressure column, the cryogenic air separation unit configuredto produce an impure argon stream having between about and 4% and 25% ofoxygen impurities from an oxygen-argon containing stream introduced fromthe lower pressure column to the argon rectification column arrangement;(b) a heat exchanger configured to warm the impure argon stream to atemperature between about 200K and 300K against a stream of the purifiedargon product or a warm compressed and purified air stream; (c) an argoncompressor configured for pressurizing the impure argon stream to apressure between about 80 psi and 120 psi; (d) a pressure swingadsorption system configured for purifying the impure argon stream tothe purified argon product by introducing the impure argon stream, thepressure swing adsorption system comprising an adsorbent bed having anadsorbent configured for adsorbing the oxygen impurities to produce apurified argon stream and a waste gas stream, wherein the adsorbent bedis subjected to an alternating cycle having an on-line phase where theimpure argon stream is purified within the adsorbent bed to produce thepurified argon stream, and an off-line phase where the adsorbentcontained in the adsorbent bed is regenerated and produces a waste gasstream; and (e) a recycle conduit disposed between the pressure swingadsorption system and the argon rectification column arrangement andconfigured for recycling the waste gas stream from the pressure swingadsorption system to the argon rectification column arrangement disposedwithin the lower pressure column; wherein the impure argon streamcomprises a portion of the ascending argon-rich vapor exiting argonrectification column arrangement. The adsorbent used in the disclosedpressure swing adsorption system may comprise carbon molecular sieves oran ion-exchanged zeolite

The argon rectification column arrangement comprises a partition wallhaving a top section, a bottom section, a first surface, and a secondsurface opposite the first surface, the partition wall disposed withinan outer shell of the lower pressure column; a plurality of masstransfer elements disposed adjacent to the first surface of thepartitioned wall; an inlet disposed proximate the bottom section of thepartition wall configured for receiving an ascending argon-oxygencontaining vapor stream; an outlet disposed proximate the top section ofthe partition wall configured for withdrawing an ascending argon-richvapor; an inlet disposed proximate the top section of the partition wallconfigured for receiving a down-flowing liquid reflux stream; an outletdisposed proximate the bottom section of the partition wall configuredfor withdrawing a descending oxygen rich liquid stream;

In various embodiments of the present invention, an argon condensingassembly is disposed within the lower pressure column at a locationabove the argon rectification column arrangement and configured tocondense the impure argon stream from the argon rectification columnarrangement to an impure liquid argon stream against a sub-cooledbottoms liquid from the higher pressure column. In such embodiments, thedown-flowing liquid stream in the argon rectification column arrangementcomprises a portion of the impure liquid argon stream from the argoncondensing assembly. Another portion of the impure liquid argon streammay be diverted from the argon condensing assembly or from a locationdownstream of the argon condensing assembly to the pressure swingadsorption system. Alternatively, in embodiments where the impure argonstream is in gaseous form the impure gaseous argon stream is generallydiverted from an upper location of the argon rectification columnarrangement and upstream of the argon condensing assembly.

In various embodiments, the heat exchanger may be the main heatexchanger of the air separation unit where the impure argon stream iswarmed against a stream of purified, compressed air or the heatexchanger may be an argon recovery heat exchanger where the impure argonstream is warmed against the purified argon stream or against a streamof purified, compressed air or both streams.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims specifically pointing outthe subject matter that Applicant regards as his invention, it isbelieved that the invention will be better understood when taken inconnection with the accompanying drawings in which:

FIG. 1 is a schematic illustration of an embodiment of an air separationplant having an air separation unit incorporating an argon rectificationcolumn and argon condensing assembly in accordance with the presentinvention;

FIG. 2 is a schematic illustration of an alternate embodiment of an airseparation plant having an air separation unit incorporating an argonrectification column and argon condensing assembly in accordance withthe present invention;

FIGS. 3a and 3b are a partial side sectional view and a top sectionalview of the divided wall column arrangement in accordance with anotherembodiment;

FIGS. 4a and 4b are a partial side sectional view and a top sectionalview of an alternate divided wall column arrangement in accordance withanother embodiment of the present invention;

FIG. 5 is a schematic illustration of a further embodiment of an airseparation plant having an air separation unit incorporating an argonrectification column and argon condensing assembly and furtherintegrated with an adsorption based argon recovery and purificationsubsystem;

FIG. 6 is a schematic illustration of one embodiment of an adsorptionbased argon refining and purification subsystem;

FIG. 7 is a schematic illustration of yet another embodiment of an airseparation plant having an air separation unit incorporating an argonrectification column and argon condensing assembly and furtherintegrated with an argon recovery and purification subsystem;

FIG. 8 is a schematic illustration of still another embodiment of an airseparation plant having an air separation unit incorporating an argonrectification column and argon condensing assembly and furtherintegrated with an argon recovery and purification system;

FIG. 9 is a schematic illustration of still another embodiment of an airseparation plant having an air separation unit incorporating an argonrectification column and argon condensing assembly and furtherintegrated with liquid based argon recovery and purification system; and

FIG. 10 is a schematic illustration of an alternate adsorption basedargon refining and purification subsystem.

For sake of clarity, the drawings may use like reference numerals forlike components shown in the different embodiments of the invention.

DETAILED DESCRIPTION

In reference to FIG. 1 and FIG. 2, an air separation plant 10 isillustrated that in a broad sense includes an incoming air purificationand compression train or subsystem 20; main heat exchange subsystem 40;and a distillation column subsystem 50. The embodiments of FIG. 1 andFIG. 2 are configured for argon rejection in a manner described in moredetail below. Alternatively, as shown in FIGS. 5-7, the air separationplant 10 may further include and an adsorption based argon refining andpurification subsystem 150 configured to recover and purify an impure orcrude argon-rich stream.

In the incoming air purification and compression train or subsystem 20shown in FIGS. 1 and 2, the incoming feed air 22 is compressed in a mainair compressor 24 and then purified in a pre-purification unit 26 toremove high boiling contaminants from the incoming feed air. Such apre-purification unit 26 typically has beds of adsorbents to adsorb suchcontaminants as water vapor, carbon dioxide, and hydrocarbons. Asdescribed in more detail below, the compressed and pre-purified feed airstream 28 is separated into oxygen-rich, nitrogen-rich, and argon-richfractions in a plurality of distillation columns including a higherpressure column 52, a lower pressure column 54, and an argonrectification column 56.

Prior to such distillation however, the compressed, pre-purified feedair stream 28 is cooled to temperatures suitable for rectificationwithin a primary or main heat exchanger 42 using refrigeration from thevarious oxygen, nitrogen and/or argon streams produced by the airseparation plant together with supplemental refrigeration generated as aresult of turbo-expansion of various streams in an upper column turbine(UCT) arrangement (shown in FIG. 2), a lower column turbine (LCT)arrangement (shown in FIG. 1), and/or a warm recycle turbine (WRT)arrangement (not shown) as is generally known to those persons skilledin the art. Finally, in the argon refining subsystem 150 of FIGS. 5-7,the argon rich fraction that is separated in the argon rectificationcolumn may be further purified or refined, as described below, toproduce product grade argon.

In the illustrated embodiment of FIG. 1, a first portion 31 of thecompressed, pre-purified feed air stream 28, resulting from thecompression and pre-purification of the incoming feed air 22 is furthercompressed in a boosted air compressor 32 and cooled in an aftercoolerto form a high pressure air stream 33 that is fed to the main heatexchanger 42. The high pressure air stream 33 forms a liquid phase or adense fluid if its pressure exceeds the critical pressure after coolingin the main heat exchanger 42. The cooled stream 34 is then split intotwo portions, with a first portion 35 being directed through anexpansion valve 36 and into the higher pressure column 52 and a secondportion 37 is expanded through another expansion valve 38 and introducedinto the lower pressure column 54. After partial traversal through mainheat exchanger 42, a second portion 39 of the compressed, pre-purifiedfeed air stream 28 is expanded through a lower column turbine 44 togenerate supplemental refrigeration. The expanded stream 45 exiting thelower column turbine 44 is then directed to the higher pressure column52.

In the illustrated embodiment of FIG. 2, a portion 39 of the compressed,pre-purified feed air stream, resulting from the compression andpre-purification of the incoming feed air, as described above, is cooledto near saturation within a primary or main heat exchanger 42 and thecooled stream 47 is subsequently directed to the base of the higherpressure column 52. A second portion 41 of the compressed, pre-purifiedfeed air stream is further compressed in a turbine-driven air compressor43 to form a high pressure air stream 46 that is also fed to the mainheat exchanger 42. After partial traversal of main heat exchanger 42,this high pressure air stream 46 is then work expanded through a turbine48 to a pressure in the range of about 1.1 to 1.5 bar. The resulting lowpressure exhaust stream 49 is then introduced into an intermediarylocation of a lower pressure column 54. Preferably, the turbine 48 isdirectly linked or coupled to the turbine-boosted air compressor 43,which absorbs the power from the turbine 48. Alternatively, it should benoted that the work of expansion may be employed for other compressionservice or used to generate electric power. The remainder 31 of the feedair is further compressed in a boosted air compressor 32 to form a highpressure air stream 33 that is fed to the main heat exchanger 42. Thehigh pressure air stream 33 forms a liquid phase or a dense fluid if itspressure exceeds the critical pressure after cooling in the main heatexchanger. In general, the resulting high pressure air stream will exitthe main heat exchanger 42 at a temperature in the range of about 93.0 Kto 103.0 K.

The high pressure liquid air stream 34 in the embodiment of FIG. 2 isthen split into two portions. The first portion 35 is directed throughexpansion valve 36 and into the higher pressure column 52, whichtypically operates at a pressure in the range of about 5.0 bar to 6.0bar. The remaining portion 37 is expanded through valve 38 andintroduced into the lower pressure column 54. In general, the highpressure air stream 34 will constitute about 25% to 35% of the total airfeed entering the air separation plant 10. In addition about 5% to 15%of the incoming air feed will be expanded in turbine 48.

It should be noted that higher pressure column 52, the lower pressurecolumn 54, and the argon rectification/rejection column 56 representdistillation columns in which vapor and liquid are counter-currentlycontacted in order to affect a gas/liquid mass-transfer based separationof the respective feed streams. Such columns will preferably employstructured packing or trays.

As shown in FIGS. 1 and 2, within the higher pressure column 52, theexpanded liquid air and gaseous air are separated into a nitrogen-richoverhead 51, a nitrogen-rich shelf draw 59 and oxygen-rich bottoms 53(i.e. kettle liquid). The condensation of a portion of the nitrogen-richoverhead 51 is effected by introducing a portion thereof asnitrogen-rich vapor stream 61A into a main condenser 60. The latent heatof condensation is imparted to the oxygen-rich bottoms 55 of the lowerpressure column 54. The resulting nitrogen rich liquid stream 62 is thendivided with a portion 63 directed to reflux higher pressure column 52while the remaining portion 64 may be subcooled and taken as liquidnitrogen product 66 via valve 65. The remaining portion of thenitrogen-rich overhead 61B may be taken via main heat exchanger 42 as agaseous nitrogen product 76. The nitrogen-rich shelf draw 59 issubcooled in subcooler 70A and the resulting subcooled stream 69 isdirected to the lower pressure column 54 via valve 71 as reflux stream.

The oxygen-rich kettle liquid stream 53 composed of the bottoms liquidof the higher pressure column 52, the shelf draw 59, and remainingportion of the liquid nitrogen stream 64 are preferably cooled againstwarming nitrogen streams 57, 58 derived or taken from lower pressurecolumn 54 within subcooler/heat exchangers 70A, 70B. The warmednitrogen-rich vapor streams 67, 68 are then directed to the main heatexchanger 42 where it is further warmed to produce a nitrogen productstream 78 and/or nitrogen waste stream 77. Although not shown, a portionof the warmed nitrogen streams often finds use as a purge/sweep fluidfor purposes of regenerating the warm end adsorbent systems of thepre-purification unit 26.

Within the lower pressure column 54, the oxygen-rich kettle liquid,liquid air stream, and nitrogen-rich shelf are further separated into anitrogen-rich overhead stream 58 and into an oxygen-rich bottoms liquid55, typically of greater than about 99.5% purity. This liquid oxygenstream 55 is extracted from the base of the lower pressure column 54 andthen elevated in pressure by a combination of gravitational head and/ormechanical pump 75. A first portion of this pressurized liquid oxygenstream 80 is split into a liquid oxygen product fraction 82 which isdirected through valve 84 into suitable storage vessel (not shown). Thisoxygen may alternatively be withdrawn before the pump. The remainingliquid oxygen fraction 86 is vaporized and warmed within main heatexchanger 42 and emerges as high pressure gaseous oxygen product stream88 that may be used directly or directed to a distribution pipeline. Inmany embodiments, the bulk of the high pressure air stream 33 isliquefied for purposes of vaporizing the liquid oxygen 86. The resultingliquid air stream 34 is distributed into the distillation column system50, as generally described above. The high pressure air 34 and pumpedoxygen 86 can be above their critical pressure. In such cases theliquefaction of the high pressure air 34 and vaporization of the liquidoxygen 86 are not discrete phase changes.

Divided Wall Argon Rectification Column

With reference to FIGS. 1-4 and particularly FIGS. 3a, 3b, 4a and 4b ,within the footprint of the lower pressure column structure, anintermediate portion of the column structure preferably contains adivided wall column arrangement 90 having a main distillation section 91and a partitioned argon rejection section 92. In the illustratedembodiments, the partitioned argon rejection section 92 is configured asan argon rectification column 56 whereas the main distillation section91 is configured as a portion of the lower pressure distillation column.It has been found that for certain air separation plants, and inparticular many gas only oxygen plants, an argon rectification columncan enable large power savings. Rejecting argon using an argonrectification column serves to increase oxygen recovery in an airseparation plant that is not typically designed to recover argon. Asdiscussed above, in many cases a separate argon rectification columninvolves high capital costs. This is especially true in larger plantsthat would require an additional or enlarged cold box package toaccommodate the separate argon rectification column.

The additional capital cost typically associated with a separate argonrejection column is greatly reduced if, as contemplated in the presentembodiments, the argon rectification column 56 is combined with anddisposed within the lower pressure column 54 structure as a divided wallcolumn arrangement 90. It is important to note that when making an argonproduct in many conventional cryogenic air separation units, a definedsection of the lower pressure column is typically under-utilized orunloaded because some of the vapor is “bypassed” to the external crudeargon or ‘superstaged’ column so that the flow area of thisunderutilized or unloaded section of the lower pressure column requiredfor distillation can be reduced and somewhat less than the flow area forthe remainder of the lower pressure column sections. As a result, anargon rectification column can be co-located in this under-utilized orunloaded section of the lower pressure column structure by designing adivided wall column having a main distillation section and a partitionedargon rejection section at this location of the lower pressure columnstructure. In such arrangement, a portion of the vapor from the adjacentsection of the lower pressure column immediately below the divided wallcolumn flows to the partitioned argon rejection section 92. Theremaining portion of the vapor from the adjacent section of the lowerpressure column immediately below the divided wall column arrangement 90flows upward through to the main distillation section 91.

The divided wall argon rectification column disposed within partitionedargon rejection section 92 of the lower pressure column structureoperates at a pressure comparable to the pressure within the lowerpressure column. The partitioned argon rejection section 92 receives anupward flowing argon and oxygen containing vapor feed 94 from the lowerpressure column, typically having a concentration of about 8% to 15% byvolume argon, and a down-flowing argon rich reflux 98 received from anargon condensing assembly 99. The partitioned argon rejection section 92serves to rectify the argon and oxygen containing vapor feed 94 byseparating argon from the oxygen into an argon enriched overhead vaporstream 95 and an oxygen-rich liquid stream 96 that that is released orreturned into the lower pressure column 54 at a point below the dividedwall column arrangement 90. The mass transfer contacting elements withinthe divided wall argon rectification column arrangement could be traysor other packing. Possible column packing arrangements includestructured packing, strip packing, or silicon carbide foam packing.

The resulting argon-rich vapor overhead stream 95 is then preferablydirected to the argon condensing assembly 99 or argon condenser alsodisposed within the structure of the lower pressure column where all ora portion of the argon-rich vapor overhead stream 95 is condensed into acrude liquid argon stream 98. The resulting crude liquid argon stream 98is used as an argon-rich reflux stream for the partitioned argonrejection section 92 and optionally taken an impure or crude liquidargon stream (not shown). In the depicted embodiments, the argon-richreflux stream 98 is directed back to the uppermost portion of thepartitioned section 92 and initiates the descending argon liquid phasethat contacts the ascending argon and oxygen containing vapor feed 94.In some alternate embodiments, a portion of the argon-rich reflux stream98 may be directed as a crude argon-rich liquid stream 98B to adownstream adsorption based argon refining and purification subsystem150 in air separation plants having specific argon product requirements.Likewise, a portion of the argon-rich vapor overhead stream 97 may bediverted and directed to the main heat exchanger 42 to recoverrefrigeration or the portion of the argon-rich vapor overhead stream 97can be diverted and directed as a crude argon-rich stream 97B to theadsorption based argon refining and purification subsystem 150.

In the illustrated embodiments, the height of the partitioned argonrejection section 92 is preferably limited to accommodate between about15 and 40 stages of separation, and more preferably between 20 and 30stages of separation. While such limited number of separation stages issufficient for argon rectification needed to improve the oxygen recoveryof the cryogenic air separation unit, the resulting purity of the argonrectification vapor stream exiting the partitioned argon rejectionsection 92 is relatively low at about 4% to 25% oxygen, and morepreferably between 10% and 15% oxygen, with up to 1% nitrogenimpurities.

FIGS. 3a and 3b show a schematic representation of a limited height,annular divided wall argon rectification column, using the outer annularspace as the argon rectification column or partitioned argon rejectionsection 92 and the inner annular space as the main distillation section91. For a limited height, annular divided wall column, trays orstructured packing can be used as mass transfer media in the partitionedsection 92 whereas structure packing is the preferred mode of separationin the main distillation section 91. As discussed above, the dividedwall argon rectification column is a partitioned section 92 disposed ina juxtaposed orientation with the main distillation section 91 bothwithin an outer shell of the lower pressure column 54. The divided wallargon rectification column is preferably an annular or cylindricalconfiguration (shown in FIGS. 3a and 3b ) but a segmented or planarconfiguration (shown in FIGS. 4a and 4b ) is equally effective. Ineither configuration, the ratio of the cross sectional area of the maindistillation section 91 to the cross sectional area of the partitionedsection 92 (i.e. argon rectification column) is between about 0.5:1 and5:1.

The partitioned section 92 of the divided wall column arrangements ofFIGS. 3a and 3b as well as the arrangements in FIGS. 4a and 4bpreferably includes a partition wall 93 having a top section, a bottomsection, a first surface, a second surface opposite the first surface,and a plurality of mass transfer elements disposed adjacent to the firstsurface of the partitioned wall forming the argon rectification column.The ascending vapor is an argon-oxygen stream 101 that enters thepartitioned argon rejection section 92 via an inlet area 102 disposedproximate the bottom section of the partition wall 93 and is directed tothe mass transfer elements such as separation trays 108. A second inletarea 104 disposed proximate the top section of the partition wall isconfigured to receive a down flowing liquid stream 103 required tofacilitate the argon rectification. The divided wall argon rectificationcolumn arrangement 90 further includes a first outlet area 105 disposedproximate the top section of the partition wall 93 for withdrawing anascending argon-rich overhead vapor 95 and a second outlet area 107disposed proximate the bottom section of the partition wall 93 forwithdrawing the descending oxygen rich liquid stream 96 and releasingthe descending oxygen rich liquid stream 96 into the lower distillationsections of the lower pressure column 54.

Similarly, the main distillation section 91 of the illustrated dividedwall column arrangements include a plurality of mass transfer elementsconfigured continue the air separation occurring within the lowerpressure column. In the preferred annular divided wall configuration ofFIGS. 3a and 3b , the annular argon region surrounds and is concentricwith the annular oxygen-nitrogen region whereas in the planar dividedwall 93 configuration of FIGS. 4a and 4b , the partitioned section 92and the main distillation section 91 are disposed in a side by sidearrangement divided by the partition wall 93.

As described in more detail below, the argon condensing assembly 99 ispreferably configured as a once-through argon condenser and is disposedinternal to the lower pressure column 54, just above the divided wallarrangement 90 of the lower pressure column structure that forms theargon rectification column. This location of the argon condensingassembly 99 or argon condenser is the natural feed point for the kettleliquid and vapor, and the natural point to condense the argon overheadvapor 95. As a result, this location is an ideal location to house theargon condenser 99 to minimizing piping and avoiding the need for aseparator vessel for the two phase partially boiled kettle stream.Alternatively, the argon condenser 99 may be disposed at the uppermostportion of lower pressure column 54, although additional piping may berequired.

Internal Argon Condenser

The illustrated embodiments provide an improved method and arrangementfor argon recovery from a cryogenic air separation unit configured witha higher pressure column 52, a lower pressure column 54 and a dividedwall argon rectification column 56. As seen therein, the improved methodand arrangement for argon recovery comprises condensing the argon-rich,overhead vapor 95 from the top of the divided wall argon rectificationcolumn in an argon condensing assembly 99 disposed at an intermediatelocation within the lower pressure column 54. In the preferredembodiment, the argon-rich overhead vapor 95 is directed to the argoncondenser 99 via line 109 and is condensed in the argon condensingassembly 99 via indirect heat exchange with the entire kettle liquidstream 53 fed from the higher pressure column 52 and subcooled insubcooler 70B. Control of this flow is preferably accomplished via flowcontrol valve 115. Alternatively, the latent heat of the argoncondensation may be imparted to only a portion of kettle liquid streamwherein the remaining kettle liquid stream may be directed into thelower pressure column.

The argon condensing assembly 99 preferably comprises one or moreonce-through argon condenser cores and disposed at an intermediatelocation within the lower pressure column 54 where the argon-richoverhead vapor 95 from the partitioned section 92 of the divided wallargon rectification column arrangement 90 flows in a counter flowarrangement against sub-cooled and lower pressure kettle liquid orbottoms liquid 53 from the higher pressure column 52. The boil-up stream112 from the argon condensing assembly 99 is a two phase (vapor/liquid)stream that is released into lower pressure column 54 for furtherrectification or separated in phase separator 114 into a vapor stream116 and liquid stream 118 prior to being released or returned to thelower pressure column 54. The condensed, argon-rich liquid 98 is removedfrom a location proximate the bottom of the argon condensing assembly 99and may be split into two portions. The main portion is fed to the topof the partitioned section 92 of the divided wall argon rectificationcolumn arrangement to provide reflux for the divided wall argonrectification column while the optional, second portion may be taken asa crude liquid argon product. A portion of the argon-rich overhead vapor95 from the partitioned section 92 of divided wall argon rectificationcolumn arrangement can also be withdrawn as crude vapor argon product97.

With the argon condenser 99 preferably disposed internal to the lowerpressure column 54, there is the opportunity to use a portion of thedown-flowing liquid within the lower pressure column 54 combined withkettle liquid 53 as the boiling side fluid in the argon condenser.However, it may be advantageous to use only kettle liquid directly herebecause the kettle liquid is normally higher in nitrogen, and thusprovides a larger temperature difference in the internal argon condenser99. However, persons skilled in the art will also recognize thatalternate liquid streams such as a condensed air stream or a liquidnitrogen stream may be used in lieu of the crude liquid oxygen stream orthe down flowing liquid as the source of refrigeration. Furthermore, theentire crude liquid oxygen stream could be fed into the lower pressurecolumn and the internal argon condenser could be situated lower in thelower pressure column, but still immediately above the partitionedsection 92 of the divided wall argon rectification column arrangement90.

As described above, prior to entering the internally disposed argoncondenser 99, the kettle liquid stream 53 is preferably subcooled withina subcooling heat exchangers 70B and 70A along with the reflux streamthrough indirect heat exchange with a nitrogen-rich vapor stream 57, 58produced in the lower pressure column 54. The warmed nitrogen-rich vaporstreams 67, 68 are then directed to the main heat exchanger 42 where itis further warmed to produce a gaseous nitrogen product stream 78 and awaste nitrogen stream 77.

Argon Rejection and Recovery

Employing the present divided wall argon rectification columnarrangement and argon condensing assembly within the shell of the lowerpressure column of a cryogenic air separation unit can enable powersavings and may also serve to increase oxygen recovery within thecryogenic air separation unit. Preferably, an impure argon-rich streamwithdrawn from the argon rectification column can be rejected or can berecovered by diverting all or a portion of the impure argon-rich streamto an adsorption based argon purification or refining subsystem 150. Insome embodiments, discussed in more detail below, an impure argon-richliquid stream can be withdrawn from the argon condensing assembly 99disposed within the lower pressure column 54 and recovered by divertinga portion of the argon-rich liquid stream to an adsorption based argonpurification or refining subsystem 150.

In the embodiment contemplating argon rejection shown in FIGS. 1 and 2,the impure argon-rich vapor stream 97 containing between about and 4%and 25% of oxygen impurities and up to about 1% nitrogen is withdrawnfrom the argon rectification column 56 and directed to the main heatexchanger 42 where the impure argon-rich stream 97 is warmed therebyproviding a portion of the refrigeration for the air separation plant10, allowing increased oxygen recovery. This particular arrangement issuitable for use in air separation plants having no specific argonproduct requirements.

In an embodiment contemplating high purity argon recovery shown in FIG.5, an impure argon-rich stream 97 is withdrawn from the argonrectification column 56 and diverted to an adsorption based argonpurification or refining subsystem 150. This particular arrangement issuitable for use in air separation plants having specific high purityargon product requirements. As seen in FIG. 5, the simplest way ofpurifying or refining the impure argon-rich stream 97 would be tocompress the impure argon-rich stream 97 after it exits the warm end ofthe main heat exchanger 42. The warmed impure argon-rich stream 97 isthen fed to an adsorption based argon purification or refining subsystem150 such as the pressure swing adsorption (PSA) system shown in FIG. 6.The resulting purified argon vapor stream 170 is then delivered to acustomer in gaseous form or liquefied and stored as high purity argonliquid in a storage vessel 160 from which liquid argon may be deliveredto the customer, as needed.

Other embodiments contemplating argon recovery shown in FIGS. 7 and 8takes the impure argon-rich stream 97B in gaseous form and directs it toan adsorption based argon purification or refining subsystem 150comprising a separate argon recovery heat exchanger 152 and a recyclingpressure swing adsorption (PSA) system 154. Alternatively, as shown inthe embodiment of FIG. 9, it is possible to take a portion of argon-richliquid stream 98B from the argon condensing assembly 99 internallydisposed within the lower pressure column 54 as the impure argon-richstream and direct it to an liquid phase adsorption based argonpurification or refining subsystem 156.

Advantageously, since the key differences between the argon rejectionarrangements and argon recovery arrangements lie outside the airseparation unit coldbox, it becomes relatively easy and not overlycapital intensive to change or retrofit the air separation plant from anargon rejection based plant to an argon recovery based plant, dependingon the near-term argon product requirements. For example, the presentarrangements for argon production would be particularly suitable for usein cryogenic air separation plants initially designed for argonrejection that can be easily modified to provide for argon recovery at alater date when the argon production requirements for the air separationplant change.

Argon Refining

In the embodiments employing argon recovery, the impure or crudeargon-rich stream 97 in gaseous form is preferably introduced into argonrefining and purification subsystem 150 having one or more adsorbentbeds containing an adsorbent that is designed to remove oxygenimpurities and optionally nitrogen impurities from the impure or crudeargon-rich stream 97. Pressure elevation of the impure argon-rich stream97 is accomplished with a compressor or pump 151. The adsorption of theimpurities produces a purified argon stream that may be delivered as apurified argon vapor stream 170. Liquefaction of the purified argonvapor stream 170 produced from the PSA system is necessary for liquidargon production. As is well known in the art, the adsorption basedargon refining or purification subsystems generally employ analternating adsorption cycle having an on-line phase where the impure orcrude argon-rich stream 97 is purified within one or more adsorbent bedsand an off-line phase where the adsorbent contained in the adsorbentbeds is regenerated through desorption of the previously adsorbedimpurities.

One such adsorption based argon refining or purification subsystem is acryogenic or liquid phase adsorption based argon refining orpurification subsystem as generally described in U.S. patent applicationSer. No. 14/192,003 filed on Feb. 27, 2014, the disclosure of which isincorporated by reference herein.

Another adsorption based argon refining or purification subsystem 150 isthe non-cryogenic adsorption based argon refining or purificationsubsystem as shown generally in FIG. 6. As seen therein, a crudeargon-rich stream 97 from distillation column system having about 4% toabout 25% by volume oxygen and up to 1% by volume nitrogen impurities ispassed through a small argon refining heat exchanger 152 to be warmed totemperature of about 200K to 300K, more preferably 250K to 300K and mostpreferably 273K to 300K. This warmed crude argon gas stream 158 is thencompressed in compressor 159 and the compressed argon stream 161 ispassed to a PSA system comprising at least two adsorption vessels 162,164 or beds and a plurality of valves 165 wherein the at least twoadsorption vessels 162, 164 or beds are configured to remove the oxygenfrom the warmed, compressed crude argon gas stream 161 in a series ofprocess steps comprising adsorption, equalization, blowdown, andpressurization.

The PSA system preferably is a carbon molecular sieve (CMS), a zeolite4A, an ion-exchanged form of zeolite 4A or other kind of zeolite basedadsorbent to remove the oxygen impurities. The typical adsorptionpressure within the vessels during adsorption steps is in the range ofabout 80 psig to about 120 psig, and preferably between about 100 psigand 110 psig, and the temperature during the adsorption operation isnear ambient temperatures. Removal of nitrogen can be accomplishedwithin in the PSA system with the inclusion of a LiX layer in theadsorption beds. Alternatively, nitrogen impurities may be removeddownstream of the PSA system using a high ratio column as a separatepurifying step. In such alternate high ratio column embodiments (SeeFIG. 8), dirty shelf nitrogen vapor is preferably used to drive the highratio column, although clean shelf vapor can be used to drive the highratio column.

A crude argon compressor 159 is preferably included upstream ofadsorption vessels 162, 164 to provide the warmed impure or crudeargon-rich stream at the proper pressure required for the adsorptionprocess. Alternatively, a liquid impure argon-rich stream may be pumpedand vaporized. The gaseous argon product can be delivered as argonproduct, or liquefied and stored as a liquid argon product while thewaste gas or blowdown gas 172 from the PSA system is preferablyrecycled. In the case of recycling, the waste gas or blowdown gas 172from the PSA system may be recycled as stream 172A back to the argonrectification column 56 of the air separation plant 10 or as recyclestream 172B back to the feed of the PSA system. In some embodiments, therecycle stream 172C may be vented.

The embodiment of the adsorption based argon refining and purificationsubsystem shown in FIG. 6 has an estimated argon recovery of about 20%.Such modest argon recovery levels may be acceptable for many airseparation plants, particularly where large gas only air separationplants are contemplated. As such modest argon recovery at low cost maybe the best economic choice. Also, this may be more suitable insituations where the merchant argon market is not expected to developuntil later. However, if a portion of the waste gas or blowdown gas 172is recycled back to the feed of the PSA system, the argon recovery inthe PSA system can be increased to about 60% or more. Enhanced recovery,however, will generally involve additional capital and operating costssuch as the use of additional adsorption beds and multiple equalizationsteps to enable even higher argon recovery. The embodiment of theadsorption based argon refining and purification subsystem 150 of FIG. 6may be incorporated within the air separation unit (ASU) schematics andflowsheets shown in FIGS. 5, 7, and 8.

In the embodiments illustrated in FIGS. 7 and 8, the impure or crudeargon-rich vapor stream is routed to a separate, small argon recoveryheat exchanger 152. A balancing warm stream 185, preferably an airstream, and a liquid nitrogen stream 59B are needed to make this heatexchange effective. These embodiments also contemplate recycling aportion of the waste gas or blowdown gas back to the argon rectificationcolumn via stream 172A, 180. Optionally, a portion of the waste gas orblowdown gas may be recycled as stream 172B back to the argon-rich feedof the PSA system 154.

In the embodiment of FIG. 7, after warming a gaseous impure or crudeargon-rich stream 97B to about ambient temperature, the warmed crudeargon-rich stream 158 is compressed or pumped via pump 151 or compressor159 to feed the adsorption beds 162, 164. The preferred operatingpressure is in the range of about 80 psig to about 120 psig, andpreferably about 110 psig, Gas buffer tanks may be useful for thisadsorption based argon refining and purification subsystem, but are notshown in FIG. 7. In order to enhance the overall argon recovery, aportion of the waste gas or blowdown gas 172A and 180 from the adsorbentbeds can be returned to the argon rectification column 56. Since theoperating pressure of the argon rectification column is low, return ofthe waste gas or blowdown gas requires little or no elevation of itspressure. While it is acceptable to return the waste gas to any locationof the argon rectification column, the preferred return point can beproximate the upper half of the argon rectification column between themiddle of the argon rectification column and the top of the argonrectification column. The recycle feed located near the middle of theargon rectification column is preferably at a location where there are asimilar number of theoretical stages above this location and below thislocation. The overall argon recovery may also be increased by recyclinga portion of the waste gas 172B and combining the recycled waste gaswith crude argon feed 97B to the adsorption based system, upstream ofthe pump or compressor. Either or both of these argon recycle streamscan be used to increase argon recovery, although the preferredarrangement recycles all or most of the waste gas or blowdown gas to theargon rejection column as stream 172A.

In FIG. 7, the adsorption beds preferably include a layer or layers ofmaterial such as LiX for essentially complete removal of the nitrogencontained in the warmed, compressed crude argon-rich stream 161. Thepurified gaseous argon product 170 exiting the adsorption beds 162, 164is very pure, and it meets the specification for oxygen and nitrogenimpurities in typical argon products (i.e. less than 1 ppm to 10 ppmoxygen, less than 1 ppm to 10 ppm nitrogen). The purified gaseous argonproduct 170 also remains at elevated pressure (e.g., about 75 psig to115 psig). After withdrawal of the purified gaseous argon product 170from the PSA system 154, it is passed into the argon recovery heatexchanger 152. Here it is cooled, condensed and subcooled against thecrude argon-rich feed stream 97B and a portion of the dirty shelf liquidstream 59B from the higher pressure column 52. The subcooled, liquidargon 174 is then reduced in pressure via expansion valve 175 and passedto an argon product storage vessel. There is often a flow imbalance thatoccurs in the argon recovery heat exchanger 152, particularly when aportion of the waste gas or blowdown gas is vented to the atmosphere asstream 172C and not recycled as streams 172A and/or 172B. That is, thereturning or recycle flow 172A in the argon recovery heat exchanger 152may be lower than the flow of the warming streams. In order tosatisfactorily warm the feed argon-rich stream 97B to near ambienttemperature and to prevent excessive refrigeration loss, an optional airbalance stream 185 is used. The optional air balance stream 185 ispreferably a diverted portion of the compressed, purified feed airstream that is directed to the argon recovery heat exchanger 152 andreturned as stream 184 to the air separation unit at a location upstreamof turbine 44.

FIG. 8 differs from FIG. 7 in that there is little or no capability forremoval of nitrogen impurity contained in the crude argon-rich feed 161to the adsorbent beds 162, 164. Without a layer or layers of nitrogenremoving adsorbent, a significant portion of nitrogen in the crudeargon-rich feed 161 passes through the PSA system. For removal ofnitrogen in this case, a high ratio argon column 190 is employed. Theelevated pressure gaseous argon product 170 is cooled in the argonrecovery heat exchanger 173 only to approximately its dew point. Thevapor argon stream 186 is then fed to a reboiler 188 at the base of thehigh ratio argon column 190. Here the argon vapor stream 186 iscondensed and withdrawn approximately at its saturated liquid state 192.The liquid stream 192 is reduced to column pressure through the feedvalve 193 and fed at the appropriate location in the high ratio argoncolumn 190. Nitrogen removal in the high ratio argon column 190 enablesproduct grade argon 195 to be withdrawn at or near its base. The productgrade argon liquid 195 through a control valve 196 prior to feed into anargon product storage vessel (not shown). Partial condensation of thenitrogen-rich overhead 191 in condenser 199 at the top of the high ratioargon column 190 can be accomplished by several cold liquid streams 197which may include shelf liquid, dirty shelf liquid, oxygen-enrichedliquid, or even liquid air. After vaporization of stream 197 in thecondenser 199, the vaporized stream 189 is combined with the wastenitrogen stream 57 from the lower pressure column 54 before it is warmedin subcooler/heat exchangers 70B and 70A. The partially condensednitrogen-rich stream 194 is phase separated in separator 19 with theliquid being returned to the high ratio column 190 as reflux and a smallvapor stream 198 that contains the nitrogen impurity removed from theargon feed stream to the column is then vented to atmosphere.

An alternative method for enhanced nitrogen removal is via an argonpasteurization section disposed proximate the top of the argonrectification column. Interposed between the argon condensing assemblyand the argon pasteurization section of the argon rectification columnis a phase separator from which a small nitrogen-rich vent stream isexhausted, with the remaining crude argon liquid directed to the argoncolumn pasteurization section as reflux for the argon rectificationcolumn. Although not shown, the argon rectification column in thisembodiment includes a distillation section and a pasteurization sectiondisposed immediately above the distillation section. A crude argonproduct stream or impure argon vapor stream is preferably removed fromthe argon rectification column near the top portion of the distillationsection and below the pasteurization section while an overhead vaporstream is removed from the argon rectification column near the topportion of the pasteurization section and directed to the argoncondensing assembly where it is partially condensed. With the argonpasteurizing section at the top of the argon rectification column, thenitrogen content of the overhead vapor stream from the argonrectification column directed to the argon condensing assembly is higherthan the crude argon product stream removed from the top portion of thedistillation section. All or a portion of the condensed crude argonliquid is then sent back to argon rectification column as reflux. Thesmall amount of remaining overhead vapor that is not condensed is thenremoved as the nitrogen-rich vent stream from a downstream phaseseparator, thus enhancing the nitrogen removal.

For the configurations schematically illustrated in FIG. 7, the highestefficiency will be when the balancing air stream 185 is returnedupstream of the lower column turbine 44. Alternatively, if the balancingair stream 185 is returned downstream of the turbine 44, but upstream ofthe higher pressure column 52, there is only a minor efficiency penalty.A larger efficiency penalty will be incurred if the balancing air stream185 is fed into the lower pressure column 54 or combined with the wastenitrogen streams 57, 67, 77 from the air separation unit. A smallportion of the dirty shelf liquid 59B is preferably withdrawn in FIGS. 7and 8 valve expanded in valve 169 and used to fully condense and subcoolthe purified gaseous argon product 170 in a section of the argonrecovery heat exchanger 152 with the vaporized shelf stream 181 exitingthe argon recovery heat exchanger 152 being directed to and combinedwith the waste nitrogen stream 57. Alternatively, clean shelf liquid oranother liquid nitrogen stream could be used to fully condense andsubcool the argon product stream in the argon recovery heat exchanger.

The configuration of FIG. 9 differs from that of FIG. 8 in that thecrude argon-rich stream is withdrawn from the argon rectification columnas a liquid stream 98B rather than as a vapor stream. Specifically, inthe embodiment of FIG. 9, a portion of the argon liquid return 98 fromthe argon condensing assembly 99 is diverted or withdrawn as theargon-rich liquid stream 98B. Alternatively, the argon rich liquidstream may be withdrawn directly from within the argon rectificationcolumn, at or near the top. A pump 182 raises the pressure of the crudeargon rich liquid stream 98B to the desired pressure for the liquidbased adsorption system 156. Alternatively, gravity head may providesufficient pressure elevation without the need for a pump. Aftervaporization and warming in the argon recovery heat exchanger 173, thepressurized crude argon rich stream 161 is purified in the adsorbentbeds 162, 164. In order to effectively vaporize and warm the crudeargon-rich stream, an elevated pressure stream 185 must be introduced inthe argon recovery heat exchanger 152. For most effective vaporizationand warming of the crude argon-rich stream, a partially cooled stream ispreferred. In FIG. 9, a minor portion of the intermediate temperaturevapor air stream 185 upstream of the lower column turbine 44 iswithdrawn and fed at the appropriate location in the argon recovery heatexchanger 173. This stream 185 is condensed and combined with the airstream 39 prior to feeding the higher pressure column 52 and the lowerpressure column 54. The elevated pressure crude argon-rich liquid ispreferably between about 95 psia and 135 psia. The intermediatetemperature air stream is preferably between 225 psia and 325 psia. Itis acceptable that the intermediate temperature air stream 185 exceedsthis pressure range if the desired pressure stream is not available.

As an alternative to the withdrawal of a portion of the intermediatetemperature air stream prior to turbine expansion, an intermediatetemperature stream from the booster air compressor may be used. Thisalternative stream may be a portion of the stream delivered at the finaldischarge pressure of the booster air compressor, or it may be a streamwithdrawn at an intermediate pressure from the booster air compressor.In the configuration of FIG. 9, adsorption materials such as LiX fornitrogen removal are not employed as the nitrogen removal isaccomplished by means of the high ratio column 190. As in the FIG. 8configuration, the purified gaseous argon product stream 170 is cooledto a near saturated vapor state in the argon recovery heat exchanger152, and then fed to the reboiler 188 of the high ratio argon column190. The configuration of the high ratio argon column 190 is similar tothat described in FIG. 8. Likewise, preferably at least a portion of thelow pressure waste stream 172C from the adsorbent beds is cooled andreturned to the argon rectification column 56, similar to that of FIGS.7 and 8. The configuration of FIG. 9 avoids the need for feedcompression of the crude argon-rich stream prior to the adsorbent beds.Optionally, a portion of the waste from the adsorbent beds may berecycled as stream 172B back to the PSA system. To accomplish this, acompressor 200 is now required to elevate the pressure of the recycledwaste stream 172B before it is combined with the warmed and vaporizedcrude argon-rich feed.

A still further embodiment of the adsorption based argon refining andpurification subsystem is shown in FIG. 10. Advantageously, theembodiment of the adsorption based argon refining and purificationsubsystem 250 shown in FIG. 10 provides enhanced argon recovery withnominal increases in capital costs and operating costs. The disclosedembodiment employs a multi-stage PSA process with appropriately sizedcommercial adsorption beds 210, 211, 220, 221, 230, and 231 operating inseries with a plurality of and control valves 217, 227, 237, tanks 216,226, 236, heat exchanger 219 and compressors 228, 238 to increaseoverall argon recovery. In such embodiment, the blowdown or wastestreams 212 and 222 of the upstream PSA stages are directed asargon-rich feed streams to one or more downstream PSA stages while theargon-enriched product streams 225 and 235 of the downstream PSA stagesare recycled back to and combined with the crude argon-rich feed stream161 to the first PSA stage. The systems and methods generally describedherein with reference to FIG. 10 may enable the adsorption based argonrefining and purification subsystem 250 to reach an argon recovery levelof more than 70%, and preferably more than 85%.

Specifically, FIG. 10 illustrates a multi-stage adsorption based argonrefining and purification subsystem 250 with three PSA stages, eachstage comprising a 2-bed PSA system. The first PSA stage of thethree-stage PSA system receives an impure or crude argon rejectionstream 161 and produces a product grade argon stream 215 which may befurther processes as product grade argon 174. The blowdown or wastestream 212 from the first 2-bed PSA stage is directed via tank 226 andcompressor 228 to a second 2-bed PSA stage. The second 2-bed PSA stageis configured to take the argon from the blowdown or waste stream 212 ofthe first 2-bed PSA stage as an argon feed and enrich it to a low gradeargon product stream having the same or similar argon concentration asthe impure or crude argon rejection stream feed directed to the first2-bed PSA stage. The size of the second 2-bed PSA stage is smaller thanthe first 2-bed PSA stage. The enriched low grade argon product stream225 produced by the second 2-bed PSA stage is recycled back to andcombined with the impure or crude argon stream 161 feed directed to thefirst 2-bed PSA stage.

Similarly, an optional third 2-bed PSA stage is configured to receivethe blowdown or waste stream 222 of the second 2-bed PSA stage via tank236 and compressor 238 and enriches it to form another low grade argonproduct stream 235 having the same or similar argon concentration ascrude argon rejection stream feed 161. Again, the size of the third2-bed PSA stage is smaller than both the first and second 2-bed PSAstages. The enriched low grade argon product stream 235 produced by thethird 2-bed PSA stage is also recycled back to the crude argon rejectionstream feed 161 directed to the first 2-bed PSA stage. Although FIG. 10shows a three stage PSA system, additional stages may be added tofurther enhance the argon recovery to well above 90%.

Examples

Process modeling has shown that using an impure or crude argon-rich feedhaving a concentration of about 90% argon and about 10% oxygenimpurities, a two stage PSA system could achieve argon recovery of 71%while a three stage PSA system shown in FIG. 10 could achieve argonrecovery of 86%. An example is shown in Table 1 to illustrate theprocess metrics in a three stage PSA process.

TABLE 1 Crude Argon-Rich Feed to PSA system at 90% Argon and 10% OxygenImpurities PSA- PSA- PSA- Stage 1 Stage 2 Stage 3 Production EnrichmentEnrichment Feed Concen- Ar % 90 88 72 tration O₂ % 10 12 28 FlowrateNCFH 1.0 0.82 0.1 Product Concen- Ar % 99.9999 90 90 tration O₂ % 0.000110 10 Flowrate NCFH 0.18 0.72 0.05 Waste Concen- Ar % 88 72 55 trationO₂ % 12 28 45 Flowrate NCFH 0.82 0.1 0.05 Process Argon % 20 90 60Recovery

In the example highlighted in Table 1, the impure or crude argon-richfeed from the distillation column is 90% argon and 10% oxygenimpurities. For easy demonstration, the impure or crude argon-rich feedflow is set at about 1.0 NCFH. As shown in Table 1, the processconditions such as concentrations and flowrates are calculated based onmodeled argon process recovery for each of the three stages in themulti-stage, adsorption based argon refining and purification subsystem.The feed stream to PSA stage 2 is the waste stream from PSA stage 1 at aconcentration of about 88% argon and 12% oxygen impurities. A compressoris required to compress this waste stream to the selected PSA systempressure of about 110 psig and a flowrate of about 0.82 NCFH. Thecompressed waste stream from the PSA stage 1 is directed to PSA stage 2.The enrichment product produced by the PSA stage 2 is about 90% argonand 10% oxygen impurities, the same as the impure or crude argon-richfeed to PSA stage 1. This low grade product stream from PSA stage 2 isat a flow rate of about 0.72 NCFH and is recycled back to and combinedwith the impure or crude argon-rich feed fresh crude feed to PSA stage1.

When the optional stage 3 is used, the feed stream to PSA stage 3 is thewaste stream from PSA stage 2 at a concentration of about 72% argon and28% oxygen impurities and a flowrate of about 0.10 NCFH. As discussed inmore detail below, this waste stream is further compressed using acompressor prior to entering PSA stage 3 beds. The argon enrichmentproduct produced by PSA stage 3 is also about 90% argon and 10% oxygenimpurities, the same as the impure or crude argon-rich feed to PSAstage 1. This low grade product stream from PSA stage 2 is at a flowrate of only about 0.05 NCFH and, like the waste stream from PSA stage 2is recycled back to and combined with the impure or crude argon-richfeed fresh crude feed to PSA stage 1. It should be noted that the argonfeed flow to PSA stage 1 in this example is constant at about 1.0 NCFHand the argon product flow from PSA stage 1 is fairly constant at about0.18 NCFH. As a result, the recovery of argon for the overall process isincreased to 86% for the three stage PSA system with the argon feedconcentration at 90% argon and 10% oxygen impurities while the overallargon recovery for a two stage PSA system at these feed conditions isabout 71%.

As indicated above, for the waste stream recycle process in amulti-stage PSA system described herein, one or more compressors 228,238 may be required to compress the waste streams and feed thedownstream adsorbent beds. Depending on the oxygen concentration in thewaste stream, extra compressor cost may be incurred for this recycleprocess, particularly where the oxygen impurity concentration is greaterthan about 23.5%. To minimize capital costs and improve the safetycharacteristics of the present adsorption based argon refining andpurification subsystem, it is desirable to avoid use of the higher costcompressors. As a result, it may be advantageous to design or configurethe argon refining and purification process to keep the oxygenconcentration in any waste stream requiring compression to aconcentration of less than about 23.5%.

As shown in Table 1, the oxygen concentration in the waste stream fromPSA stage 1 in the above example is only about 12%, so a standardcompressor design is sufficient for this waste stream in the multi-stagePSA system and process. However, the waste stream from PSA stage 2 hasan oxygen concentration of about 28%, which means a more expensivecompressor may be needed if this waste stream is to be safely directedto PSA stage 3. Although additional stages of the multi-stage PSA systemor arrangement will enable higher argon recoveries, the additionalcapital costs for additional stages may adversely impact the economicsof the argon refining and purification process. In the present exampleshown in Table 1, the flow of waste stream from PSA stage 2 to PSA stage3 is only about 10% of the impure or crude argon-rich feed flow to themulti-stage PSA system. Thus, it may be more economical to recycle thiswaste stream back to argon rectification column to recover argon.

Another example of the present multi-stage adsorption based argonrefining and purification subsystem with three PSA stages, each stagecomprising a 2-bed PSA system is provided in Table 2. This example showsthe performance of multi-stage adsorption based argon refining andpurification subsystem of FIG. 10 with a slightly lower argonconcentration in the impure or crude argon-rich feed coming from argonrejection column, namely an argon concentration of about 85% and anoxygen impurity concentration of about 15%. As expected, the resultsshown in Table 2 indicate that the higher oxygen impurity concentrationin the argon feed will generate higher oxygen concentrations in thewaste streams. In this example, a two stage PSA with a standard normalair compressor still can be used for the waste stream recycle and stillprovide about 71% overall argon recovery whereas with a three stage PSAsystem the overall argon recovery remains at about 86%.

TABLE 2 Crude Argon-Rich Feed to PSA system at 85% Argon and 15% OxygenImpurities PSA- PSA- PSA- Stage 1 Stage 2 Stage 3 Production EnrichmentEnrichment Feed Concen- Ar % 85 82 62 tration O₂ % 15 18 38 FlowrateNCFH 1.0 0.83 0.11 Product Concen- Ar % 99.9999 85 85 tration O₂ %0.0001 15 15 Flowrate NCFH 0.17 0.72 0.05 Waste Concen- Ar % 82 62 44tration O₂ % 18 38 56 Flowrate NCFH 0.83 0.11 0.06 Process Argon % 70 9060 Recovery

The improved PSA system argon recoveries of the FIG. 10 configurationmay allow satisfactory argon production without the need for furtherrecycling. However, improved recovery PSA systems provide a largebenefit in combination with recycling of the waste gas to the argonrectification column in order to enable even higher argon production.The higher characteristic recovery of the PSA system greatly reduces theflow of the recycling argon. That is, the return flow of the waste gasand the flow of the crude argon-rich product are reduced when the PSAsystem can achieve higher recovery. For example, a PSA system recoveryof 60% will reduce these flows nominally by a factor of three comparedto a PSA system recovery of 20% when all the waste gas is recycled tothe argon rectification column. This provides significant advantage tothe system. The lower flows greatly reduce the capital cost of the feedcompressor and associated operating costs as a result of its lower powerconsumption. The lower flows also mean the adsorbent beds and theassociated piping and valves may also be smaller and less expensive. Thelower recycling flow further reduces the effect of the waste gas on thedesign of the argon rectification column and argon condenser.

While the present invention has been described with reference to apreferred embodiment or embodiments and operating methods associatedtherewith, it is understood that numerous additions, changes andomissions to the disclosed systems and methods can be made withoutdeparting from the spirit and scope of the present invention as setforth in the appended claims.

What is claimed is:
 1. A method of producing a purified argon product ina cryogenic air separation unit integrated with a pressure swingadsorption system, the method comprising the steps of: separating argonfrom an oxygen-argon containing stream within a lower pressure column ofthe cryogenic air separation unit using an argon rectification columnarrangement disposed within the lower pressure column, the separation ofthe argon from the oxygen-argon containing stream producing an impureargon stream having between about and 4% and 25% of oxygen impurities;warming the impure argon stream; pressurizing the impure argon stream;purifying the impure argon stream by introducing the impure argon streaminto the pressure swing adsorption system, the pressure swing adsorptionsystem comprising an adsorbent bed having an adsorbent configured foradsorbing the oxygen impurities to produce a purified argon stream and awaste gas stream, wherein the adsorbent bed is subjected to analternating cycle having an on-line phase where the impure argon streamis purified within the adsorbent bed to produce the purified argonstream, and an off-line phase where the adsorbent contained in theadsorbent bed is regenerated and produces a waste gas stream; andrecycling the waste gas stream from the pressure swing adsorption systemback to the argon rectification column arrangement disposed within thelower pressure column; wherein the argon rectification columnarrangement comprises a partition wall having a top section, a bottomsection, a first surface, and a second surface opposite the firstsurface, the partition wall disposed within an outer shell of the lowerpressure column; a plurality of mass transfer elements disposed adjacentto the first surface of the partitioned wall; an inlet disposedproximate the bottom section of the partition wall for receiving anascending argon-oxygen containing vapor stream; an outlet disposedproximate the top section of the partition wall for withdrawing anascending argon-rich vapor; an inlet disposed proximate the top sectionof the partition wall for receiving a down-flowing liquid reflux stream;an outlet disposed proximate the bottom section of the partition wallfor withdrawing a descending oxygen rich liquid stream; and wherein theimpure argon stream consists essentially of a portion of the ascendingargon-rich vapor exiting argon rectification column.
 2. The method ofclaim 1, wherein an argon condensing assembly is disposed within thelower pressure column at a location above the argon rectificationcolumn.
 3. The method of claim 2, wherein the down-flowing liquid streamin the argon rectification column is a portion of the liquid argon fromthe argon condensing assembly.
 4. The method of claim 3, wherein theimpure argon stream is another portion of the liquid argon streamdiverted from the argon condensing assembly or from a locationdownstream of the argon condensing assembly.
 5. The method of claim 2,wherein the impure argon stream is an impure gaseous argon streamdiverted from an upper location of the argon rectification columnarrangement and upstream of the argon condensing assembly.
 6. The methodof claim 1, wherein the adsorbent comprises carbon molecular sieves. 7.The method of claim 1 wherein the adsorbent comprises an ion-exchangedzeolite.
 8. The method of claim 1, further comprising the step ofdirecting the purified argon stream to a heat exchanger for liquefactionand recovery of the refrigeration energy.
 9. The method of claim 1,wherein the step of warming the impure argon stream to a temperaturebetween about 200K and 300K further comprises warming the impure argonstream in a main heat exchanger of the air separation unit against astream of purified, compressed air.
 10. The method of claim 1, whereinthe step of warming the impure argon stream to a temperature betweenabout 200K and 300K further comprises warming the impure argon stream inan argon recovery heat exchanger against the purified argon stream. 11.The method of claim 1, wherein the step of warming the impure argonstream to a temperature between about 200K and 300K further compriseswarming the impure argon stream in an argon recovery heat exchangeragainst the purified argon stream or against a stream of purified,compressed air or both.
 12. An apparatus for producing a purified argonproduct comprising: a cryogenic air separation unit having a higherpressure column and a lower pressure column, and an argon rectificationcolumn arrangement disposed within the lower pressure column, thecryogenic air separation unit configured to produce an impure argonstream having between about and 4% and 25% of oxygen impurities from anoxygen-argon containing stream introduced from the lower pressure columnto the argon rectification column arrangement; a heat exchangerconfigured to warm the impure argon stream against a stream of thepurified argon product or a warm compressed and purified air stream; anargon compressor or pump configured for pressurizing the impure argonstream; a pressure swing adsorption system configured for purifying theimpure argon stream to the purified argon product by introducing theimpure argon stream, the pressure swing adsorption system comprising anadsorbent bed having an adsorbent configured for adsorbing the oxygenimpurities to produce a purified argon stream and a waste gas stream,wherein the adsorbent bed is subjected to an alternating cycle having anon-line phase where the impure argon stream is purified within theadsorbent bed to produce the purified argon stream, and an off-linephase where the adsorbent contained in the adsorbent bed is regeneratedand produces a waste gas stream; and a recycle conduit disposed betweenthe pressure swing adsorption system and the argon rectification columnarrangement and configured for recycling the waste gas stream from thepressure swing adsorption system to the argon rectification columnarrangement disposed within the lower pressure column; wherein the argonrectification column arrangement comprises a partition wall having a topsection, a bottom section, a first surface, and a second surfaceopposite the first surface, the partition wall disposed within an outershell of the lower pressure column; a plurality of mass transferelements disposed adjacent to the first surface of the partitioned wall;an inlet disposed proximate the bottom section of the partition wallconfigured for receiving an ascending argon-oxygen containing vaporstream; an outlet disposed proximate the top section of the partitionwall configured for withdrawing an ascending argon-rich vapor; an inletdisposed proximate the top section of the partition wall configured forreceiving a down-flowing liquid reflux stream; an outlet disposedproximate the bottom section of the partition wall configured forwithdrawing a descending oxygen rich liquid stream; and wherein theimpure argon stream comprises a portion of the ascending argon-richvapor exiting argon rectification column arrangement.
 13. The apparatusof claim 12, further comprising an argon condensing assembly disposedwithin the lower pressure column at a location above the argonrectification column arrangement and configured to condense the impureargon stream from the argon rectification column arrangement to animpure liquid argon stream against a sub-cooled bottoms liquid from thehigher pressure column.
 14. The apparatus of claim 13, wherein thedown-flowing liquid stream in the argon rectification column arrangementcomprises a portion of the impure liquid argon stream from the argoncondensing assembly.
 15. The apparatus of claim 14, wherein the impureargon stream is another portion of the impure liquid argon streamdiverted from the argon condensing assembly or from a locationdownstream of the argon condensing assembly.
 16. The apparatus of claim13, wherein the impure argon stream is an impure gaseous argon streamdiverted from an upper location of the argon rectification columnarrangement and upstream of the argon condensing assembly.
 17. Theapparatus of claim 12, wherein the adsorbent comprises carbon molecularsieves.
 18. The apparatus of claim 12, wherein the adsorbent comprisesan ion-exchanged zeolite.
 19. The apparatus of claim 12, wherein theheat exchanger is a main heat exchanger of the air separation unit andthe impure argon stream is warmed against a stream of purified,compressed air.
 20. The apparatus of claim 12, wherein the heatexchanger is an argon recovery heat exchanger and the impure argonstream is warmed against the purified argon stream or against a streamof purified, compressed air or both streams.